Note: Throughout
this handbook, technical terms and acronyms shown in italics are defined
in the Glossary, Appendix B.

INTRODUCTION

There was
a time when radio communication was one of a few methods for instant communication
across distances. We’ve all seen black-and-white wartime film clips of
radio operators sending Morse code using bulky radio equipment. After World
War II, the communications industry turned its attention to other technologies,
leading to a period of slow growth in high-frequency (HF) radio communications
during the 1960s and 1970s.However,
HF, also known as short wave, is now undergoing an exciting revival propelled
by an infusion of new technology.

Genesis

Modern radio
technology had its birth with the publication of James Clerk Maxwell’s
Treatise on Electricity and Magnetism in 1873, setting forth the basic
theory of electromagnetic wave propagation. But the first radio waves were
actually detected 15 years later. In 1888, Heinrich Rudolph Hertz (the
scientist for whom the unit of frequency is named) demonstrated that disturbances
generated by a spark coil showed the characteristics of Maxwell’s radio
waves. His work inspired Guglielmo Marconi’s early experiments with wireless
telegraphy using Morse code. By 1896, Marconi had communicated messages
over distances of a few kilometers. It was thought at the time that radio
waves in the atmosphere traveled in straight lines and that they therefore
would not be useful for over-the-horizon communication. That opinion did
not discourage Marconi, however, who became the first to demonstrate the
transmission of radio waves over long distances. In 1901 in Newfoundland,
Canada, he detected a telegraphic signal transmitted from Cornwall, England,
3,000 kilometers away. For an antenna, he used a wire 120 meters long,
held aloft by a simple kite. Marconi’s success stimulated an intensive
effort to explain and exploit his discovery. The question of how radio
waves could be received around the surface of the earth was eventually
answered by Edward Appleton. It was this British physicist who discovered
that a blanket of electrically charged, or “ionized,” particles in the
earth’s atmosphere (the ionosphere) were capable of reflecting radio waves.
By the 1920s, scientists had applied this theory and developed ways to
measure and predict the refractive properties of the ionosphere.

Growth

In time, the
characteristics of HF radio propagation became better understood. Operators
learned, for example, that usable frequencies varied considerably with
time of day and season. HF technology developed quickly. By World War II,
HF radio was the primary means of long-haul communications for military
commanders because it provided communications with land, sea, and air forces.
In the hands of a skilled operator, armed with years of experience and
an understanding of the propagating effects of the ionosphere, HF radio
was routinely providing reliable, effective links over many thousands of
miles. Today, HF radio plays an important role in allowing emerging nations
to establish a national communications system quickly and inexpensively.

Hiatus

The advent
of long-range communications by satellite in the 1960s initiated a period
of declining interest in HF radio. Satellites carried more channels and
could handle data transmission at higher speeds. Additionally, satellite
links seemed to eliminate the need for highly trained operators. As long-range
communications traffic migrated to satellites, HF was often relegated to
a backup role. The result was user preference for wider bandwidth methods
of communication, such as satellites, resulting in declining proficiency
in HF as the number of experienced radio operators decreased. It became
clear over time, however, that satellites (for all their advantages) had
significant limitations. Military users became increasingly concerned about
the vulnerability of satellites to jamming and physical damage, and questioned
the wisdom of depending exclusively on them. Moreover, satellites and theirsupporting
infrastructure are expensive to build and maintain.

Revival

In the last
decade, we’ve seen a resurgence in HF radio. Research and development activity
has intensified, and a new generation of automated HF equipment has appeared.
These systems provide dramatic improvements in link reliability and connectivity,
while eliminating the tedious manual operating procedures required to use
older generation equipment. Today’s adaptive HF radios are as easy to use
as wireless telephones. Nonetheless, the perception that HF radio is an
inherently difficult medium continues to linger. This perception continues
only because some communicators remember how HF used to be. As your interest
in this book shows, however, HF is again being recognized as a robust and
highly competitive medium for long-haul communications, offering myriad
capabilities. In this introduction to HF radio communications, we present
information that will help you understand modern HF radio technology. We’ll
cover the principles of HF radio, talk about specific applications, and
then, consider the future of HF radio communication.